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Calcein, also known as fluorexon, fluorescein complex, is a fluorescent calcium indicator with an excitation and emission wavelengths of 495 and 515nm respectively. Calcein self-quenches at concentrations above 100 mM. It is used as a complexometric indicator for titration of calcium ions with EDTA, and for fluorimetric determination of calcium. It is slightly soluble in water. The proton dissociation constants are reported to be pKa1=2.1, pKa2=2.9, pKa3=4.2, pKa4=5.5, pKa5=10.8 and pKa6=11.7. Calcein solution emits a yellowish-green fluorescence in acidic conditions, and no fluorescence in basic conditions. However, calcein emits fluorescence in the presence of metal ions such as Al, Ba, Cu, Mg and Zn in basic conditions. Therefore, calcein can also be used for direct fluorimetric titration of these heavy metal ions besides Ca.